In a typical surveillance camera system having an auto-iris capability, the brightness of the video picture is a function of the average video level of the video signal output from the camera. To automatically control the brightness of the video picture, an auto-iris control circuit develops an auto-iris control signal which controls an adjustable iris of the camera. The auto-iris control circuit develops the control signal based upon a comparison of the average video level of the video signal output from the camera and a factory preset reference level.
When the average video level is determined to be higher than the factory preset reference level, the control signal causes the adjustable iris to decrease the amount of light reaching the camera, thereby decreasing the average video level of the video signal output from the camera. Conversely, when the average video level is determined to be lower than the factory preset reference level, the control signal causes the adjustable iris to increase the amount of light reaching the camera, thereby increasing the average video level of the video signal output from the camera. In this way, the average video level of the video signal output is maintained nearly equal to the factory preset reference level.
The auto-iris control circuit described above provides acceptable results over a wide range of illumination levels, provided the entire picture is evenly illuminated. However, whet the subject of the picture is much darker than the background of the picture, the subject may appear as a silhouette or may not be visible at all. For example, when the surveillance camera is aimed at the front license plate of a vehicle having its headlights on, due to the bright headlights, the average video level of the output of the camera exceeds the factory preset reference level. Accordingly, the control signal of the auto-iris circuit controls the adjustable iris to reduce the average video level to a level equal to the factory preset reference level. However, at such a level, an insufficient amount of light is received from the license plate to render the license plate number visible.
The opposite problem occurs when the subject of the picture is much brighter than the background of the picture. For example, when a surveillance camera in a dimly lit warehouse is viewing a brightly lighted subject located at a loading door, to average video level output from the camera is less than the factory preset reference level. As a result, the control signal of the auto-iris circuit controls the adjustable iris so as to increase the average video level of the camera's output to a level equal to the factory preset reference level. However, at such a level, the adjustable iris permits too much light to be received from the subject, and therefore, the subject appears over-illuminated.
In order to accomodate the above uneven lighting conditions in a surveillance camera with auto-iris control, the camera is usually provided with a manual override mode. When in the manual override mode, the auto-iris feature is disabled and the adjustable iris can be manually controlled from a remote control panel. While the manual override mode allows the user to set the adjustable iris as desired to overcome uneven lighting conditions, it also requires that the user manually readjust the iris each time the lighting conditions change. As a result, if the camera is inadvertently left in the manual override mode, the iris will no longer automatically react to the changing lighting conditions and therefore, subsequent surveillance operations may be made with an undesirable iris setting.
While surveillance type cameras have used the above manual mode technique to compensate for uneven lighting conditions, video cameras used in the consumer field, i.e., so-called "camcorders", have attempted to treat at least the silhouette problem in a different way. In such camcorders, a fixed level "backlighting compensation" switch is provided, which when set, causes the setting of the aperture of the iris to be increased a fixed amount over the aperture setting which is automatically set by the auto-iris control signal based on a comparison of the average video level and a factory preset reference level. However, such camcorders only provide a single compensation setting, which may or may not be appropriate for the given lighting condition. Further, the camcorder does not provide compensation for an over-illuminated condition.
The typical surveillance camera with auto-iris control as described above has a further disadvantage in that the auto-iris control circuit does not know when the adjustable iris is at its maximum setting or minimum setting position. Therefore, under certain conditions, the auto-iris control circuit attempts to drive the adjustable iris beyond its maximum and minimum operable positions. Accordingly, to prevent damage to the adjustable iris or drive motor due to this excessive driving by the auto-iris control circuit, the adjustable iris is required to contain a complicated slip clutch mechanism or limit switches.
It is, therefore, a primary object of the present invention to provide an improved auto-iris control for a surveillance camera.
It is a further object of the present invention to provide an auto-iris control for a surveillance camera which permits manual adjustment of the average video level of the video signal output from the camera while maintaining the auto-iris control.
It is a still further object of the present invention to provide an auto-iris control circuit which prevents the adjustable iris from being driven beyond its maximum and minimum operable positions.